Method for determining a distance between a vehicle and a vehicle access and starter identifier

ABSTRACT

A method for measuring a distance R separating a vehicle and an identifier for accessing and starting the vehicle includes N sequences Sq p , p∈[1;N]. Each sequence Sq p  includes: transmission, from a transmitter of the vehicle to a receiver of the identifier, of a first signal of frequency f p ; measurement, by a computer of the identifier, of a phase ϕ vip , modulo 2π, of the received first signal, in relation to a second signal of phase ϕ ip (t)=ϕ 0ip +2πf p t; transmission, from a transmitter of the identifier to a receiver of the vehicle, of the second signal; and calculation, by a computer of the vehicle, of a phase ϕ ivp , modulo 2π, of the received second signal, in relation to the first signal; calculation of the average ϕ p  of the phase ϕ vip  and of the phase ϕ ivp .

TECHNICAL FIELD OF THE INVENTION

The technical field of the invention is, in general, hands-free access and starting systems for vehicles. The invention relates more particularly to a method for determining a distance separating a vehicle from a hands-free identifier that makes it possible to access the vehicle and/or to start the vehicle.

PRIOR ART

What are termed “hands-free” access and starting systems, allowing the doors of a vehicle to be locked and unlocked and the vehicle engine to be started without the use of a traditional key, are nowadays widespread on the market.

Conventionally, when a user wishing to unlock a door of a vehicle touches a capacitive sensor or is detected by an infrared sensor situated on the door handle, a central computer of the vehicle triggers the transmission of a low-frequency (between 20 and 150 kHz) interrogation signal by a low-frequency antenna of the vehicle. As an alternative, the low-frequency antenna may send such low-frequency interrogation signals periodically (reference is made to “polling”). If an identifier (which conventionally takes the form of a key or an electronic card, or even a smartphone having a suitable activated application) in the proximity of the vehicle captures an interrogation signal, it responds by sending an unlocking code to the central computer by radio signal. A radio receiver of the vehicle then receives the radio signal: if the locking code is recognized by the central computer, then the latter orders the unlocking of the door.

The method is substantially the same when the user wishes to start the vehicle and presses a switch situated in the passenger compartment: in this case, the engine is started by the central computer only if a starting code sent by the identifier is recognized by the central computer.

To increase the security of hands-free access and starting systems, it is desirable for additional conditions to be met before locking, unlocking or starting is triggered. It is desirable in particular for the location of the identifier to be in keeping with the action to be performed, for example:

-   -   For unlocking, the identifier should be located within a maximum         perimeter, for example of 2 meters, around the vehicle     -   For locking, there should be no identifier located inside the         passenger compartment     -   For starting, the identifier should be located inside the         passenger compartment.

Thus, a reliable measurement of distance between the identifier and the vehicle is necessary.

Generally, the distance is calculated on the basis of a measurement of power (which measurement is called RSSI for “Received Signal Strength Indication”) of a low-frequency signal received by the identifier, for example the interrogation signal. Using a low-frequency signal to measure the distance is advantageous. Specifically, the vehicle-identifier distance of an identifier in the proximity of the vehicle is far shorter than the wavelength of the low-frequency signal. Refraction phenomena are thus limited. In addition, modern low-frequency receivers have a very low consumption in standby mode.

The identifier is able for example to measure the power and then transfer the measurement by radio signal to the radio receiver of the vehicle, the central computer then taking on the task of measuring the distance separating the vehicle from the identifier. As an alternative, the identifier may itself calculate the distance and respond to the interrogation signal only if this distance is shorter than a predetermined threshold (or else respond that the calculated distance does not meet a security condition).

However, this measurement technique does not take account of the phenomena of reflection and refraction that are inherent to the propagation of the low-frequency signal. Specifically, the measured power is not that of the signal on the direct path between the vehicle and the identifier, but that of a multitude of signals received by the identifier, each signal having taken a different path between the vehicle and the identifier due to the nearby reflective and refractive surfaces.

GENERAL DESCRIPTION OF THE INVENTION

The aim of the invention is therefore to propose a method for measuring distance between a vehicle and an identifier, taking account of the phenomena of reflection and refraction of the signals that may be exchanged between said vehicle and said identifier.

To this end, the invention proposes a method for measuring a distance R separating a vehicle and an identifier for accessing and starting the vehicle, the vehicle and the identifier being synchronized, the method comprising N sequences Sq_(p), p∈[1;N], N being a natural number at least greater than 3, each sequence Sq_(p) comprising the following steps:

-   -   transmission, from a transmitter of the vehicle to a receiver of         the identifier, of a first signal of frequency f_(p)     -   measurement, by a computer of the identifier, of a phase         φ_(vip), modulo 2π, of the received signal, in relation to a         second signal of phase φ_(ip)(t)=φ_(0ip)+2πf_(p)t     -   transmission, from a transmitter of the identifier to a receiver         of the vehicle, of the second signal     -   measurement, by a computer of the vehicle, of a phase φ_(ivp),         modulo 2π, of the received second signal, in relation to the         first signal     -   calculation of the average φ_(p) of the phase φ_(vip) and of the         phase φ_(ivp)         the method furthermore including:     -   for each value of p between 1 and N−1, calculation of a gradient         P_(p) using the formula P_(p)=(φ_(p+1)−φ_(p))/(f_(p+1)−f_(p))     -   filtering of the N−1 values of calculated gradients P_(p) so as         to determine an optimum gradient P     -   calculation of the distance R on the basis of the optimum         gradient P.

The term “synchronized” is understood to mean being able to implement the method.

According to the invention, a plurality of first signals and second signals are exchanged between the vehicle and the identifier. More precisely, N outward-return trips (an outward trip corresponding to a transmission of a first signal from the vehicle to the identifier, a return trip corresponding to a transmission of a second signal from the identifier to the vehicle) are performed. An average phase φ_(p) is associated with each outward-return trip, each pair of average phases (φ_(p), φ_(p+1)) making it possible to calculate a gradient P_(p). The step of filtering the gradients P_(p) then makes it possible to eliminate the outward-return trips whose signals have undergone excessively large reflections and/or refractions, and to obtain an optimum gradient P for the calculation of the distance R.

Besides the features that have just been outlined in the previous paragraph, the method according to the invention may have one or more additional features from among the following, which are considered individually or in any technically feasible combination:

-   -   the distance R is calculated, during the calculation step, using         the following formula:         R=−c/(2π)*P     -    where c is the speed of propagation of radio waves in air.     -   the N sequences Sq_(p), p∈[1;N] are such that, for all values of         p between 1 and N−1, f_(p+1)−f_(p) is constant.     -   the filtering comprises selecting a plurality of gradients from         among the N gradients P_(p) calculated during the calculation         steps.     -   the filtering comprises averaging the plurality of selected         gradients.

The invention and its various applications will be better understood upon reading the following description and upon examining the figures accompanying it.

BRIEF DESCRIPTION OF THE FIGURES

The figures are presented only by way of entirely nonlimiting indication of the invention. In the figures:

FIG. 1 shows two transceiver devices belonging to a vehicle and an identifier, respectively, between which it is desired to know the distance, the devices being designed to implement a method according to one embodiment of the invention;

FIG. 2 shows a block diagram showing steps of the method;

FIG. 3 shows signals exchanged between the transceiver devices during one step of the method;

FIG. 4 shows a graph depicting sections of average phases, modulo 2π, calculated during steps of the method, as a function of frequencies of signals exchanged during other steps of the method;

FIG. 5 shows a graph depicting the sections of FIG. 4 placed end to end.

DETAILED DESCRIPTION OF AT LEAST ONE EMBODIMENT OF THE INVENTION

Unless indicated otherwise, one and the same element appearing in different figures has a single reference.

The method described hereinafter makes it possible to calculate a distance R between a vehicle V and what is termed a hands-free identifier I, said identifier I making it possible to control, using a “hands-free” principle, access to or starting of the vehicle V. The identifier I is for example an electronic key or card, or a smartphone having a suitable application.

The vehicle V includes a first transceiver device Dv, and the identifier I includes a second transceiver device Di. As the first transceiver device Dv and the second transceiver device Di are similar, a general description is given hereinafter.

With reference to FIG. 1, a transceiver device Dz, the index z indiscriminately being v or i, includes:

-   -   a transmitter TXz of radio signals (with a frequency at least         equal to 1 GHz)     -   a receiver RXz of radio signals (with a frequency at least equal         to 1 GHz)     -   an antenna Atz to which the transmitter TXz and the receiver RXz         are connected     -   a phase-locked loop PLLz for supplying signals of various         frequencies to the transmitter TXz and to a computer Xv     -   the computer Xz for calculating the phases of signals received         by the receiver RXz in relation to signals supplied by the         phase-locked loop PLLz.

It is noted that a smartphone natively has all of the components of the described transceiver device Di. In one preferred embodiment, the identifier I is therefore a smartphone having a suitable application for the hands-free accessing and starting of the vehicle. The various components of the transceiver device Di are advantageously triggered and controlled by the application installed on the smartphone.

The method according to the invention is implemented by the first transceiver device Dv and the second transceiver device Di. It is noted that the first transceiver device Dv and the second transceiver device Di have been synchronized with one another beforehand, for example via a Bluetooth Low Energy protocol (it is noted that a smartphone natively has a Bluetooth chip).

With reference to FIG. 2, the method METH first of all includes a succession of N sequences Sq_(p), p∈[1;N], N being a natural number at least greater than 3, each sequence Sq_(p) comprising the following steps:

In a first step Em_S_(vp) of a sequence Sq_(p), at a time t_(0p), a first non-modulated signal S_(vp) of phase φ_(0vp) and of frequency f_(p) and generated beforehand by the phase-locked loop PLLv of the vehicle V, is transmitted by the transmitter TXv of the vehicle V. By way of illustration, signals S_(vp) and S_(v(p+1)), which correspond to sequences Sq_(p) and Sq_(p+1), respectively, are shown in FIG. 3. The first signal S_(vp) is received by the receiver RXi of the identifier I at a time t_(0p)+Δt=t_(0p)+R/c, where c is the propagation speed of the signal and R is the distance separating the vehicle V from the identifier I.

In a second step Mes_φ_(vip) of a sequence Sq_(p), the computer Xi of the identifier I measures the phase φ_(vip), modulo 2π, of the received first signal S_(vp), in relation to a second signal S_(ip). The second signal S_(ip) is a signal generated by the phase-locked loop PLLi of the identifier I, such that its phase has a value of φ_(0ip) and its frequency f_(p).

The measured phase φ_(vip) is able to be calculated using the following formula: φ_(vip) +k*2π=φ_(0vp)+2πf _(p) t _(0p)−φ_(0ip)−2πf _(p)(t _(0p) +R/c) k being a positive natural number.

In a third step Em_S_(ip) of a sequence Sq_(p), at a time t_(1p), the second signal S_(ip) is transmitted by the transmitter TXi of the identifier I. The second signal S_(ip) is received by the receiver RXv of the vehicle V at a time t_(1p)+Δt=t_(1p)+R/c.

In a fourth step Mes_φ_(ivp) of a sequence Sq_(p), the computer Xv of the vehicle V measures the phase φ_(ivp), modulo 2π, of the received second signal S_(ip), in relation to the first signal S_(vp).

The measured phase φ_(ivp) is able to be calculated using the following formula: φ_(ivp) +m*2π=φ_(0ip)+2πf _(p) t _(1p)−φ_(0vp)−2πf _(p)(t _(1p) +R/c) m being a positive natural number.

In a fifth step Tr_φ_(vip) of a sequence Sq_(p), the transmitter TXi of the identifier I transmits, to the receiver RXv of the vehicle V, the phase φ_(vip) that it measured previously.

In a sixth step Cal_φ_(p) of a sequence Sq_(p), a computer of the vehicle V calculates the average φ_(p) of the phase φ_(ivp) (measured in the fourth step Mes_φ_(ivp)) and of the phase φ_(vip) (received in the fifth step Tr_φ_(vip)).

The average phase φ_(p) is calculated using the following formula:

φ_(p) = (φ_(0 vp) + 2 π f_(p)t_(0 p) − φ_(0 ip) − 2 π f_(p)t_( 0p) − 2 π f_(p)R/c + φ_(0 ip) + 2 π f_(p)t_(1 p) − φ_(0 vp) − 2 π f_(p)t_(1 p) − 2 π f_(p)R/c − k * 2 π − m * 2 π)/2 = −{4 π f_(p)R/c + (k + m) * 2 π}/2

It is noted that each sequence Sqp, p∈[1;N], is associated with a frequency fp. In one preferred embodiment, the N sequences Sqp are such that, for all values of p between 1 and N−1, fp+1−fp is positive and constant. The method METH includes for example 80 sequences Sqp, such that f1=2.4 GHz, f80=2.480 GHz and, for all values of p between 1 and 79, fp+1−fp=1 MHz. Specifically, these frequencies correspond to the Bluetooth Low Energy band.

As an alternative, the sequences Sq_(p) could not comprise the fifth step, but comprise a step, between the fourth and the sixth step, of transmission, from the transmitter TXv of the vehicle V to the receiver RXi of the identifier I, of the phases φ_(ivp) measured previously. The sixth step would thus be implemented by the computer of the identifier I, and not by the computer of the vehicle V.

With reference to FIG. 2, the method METH also includes the following steps, performed following the implementation of the N sequences Sq_(p).

In a step Cal_P_(p), for each value of p between 1 and N−1, a gradient P_(p) is calculated by the computer Xv of the vehicle V using the following formula: P _(P)=φ_(P+1)−φ_(P))/(f _(P+1) −f _(P)). It is noted that: P=[−{4πf _(p+1) R/c+(k+m)*2π}/2+{4πf _(p) R/c+(k+m)*2π}/2]/(f _(p+1) −f _(p))=2πR/c.

As shown in FIG. 4, which shows the various average phases φ_(p) modulo 2π (calculated during the sixth steps Cal_φ_(p) of the sequences Sq_(p)) as a function of the various frequencies f_(p), the gradients P_(p) correspond to the incline of the segments G_(p) whose edges are the points with coordinates (f_(p); φ_(p)) and (f_(p+1); φ_(p+1)).

FIG. 5 shows the segments G₁, . . . G_(N−1) of FIG. 4 placed end to end, and a theoretical straight line Dt. The theoretical straight line Dt corresponds to an ideal case in which the first signals S_(vp) and second signals S_(ip) transmitted between the vehicle V and the identifier I do not undergo reflection or refraction: only signals on direct paths are received by the receivers Rxv, RXi of the vehicle V and of the identifier I. In a real case, the signals received by the receivers Rxv, RXi of the vehicle V and of the identifier I are not exactly the first signals S_(vp) and second signals S_(ip), but accumulations of the direct signals and of the reflected/refracted signals.

Using a single gradient P_(p) as a basis for calculating the distance R via the preceding formula therefore exhibits a non-negligible risk of error. Thus, in a step Fil_P_(p), the N−1 calculated gradient P_(p) values are filtered so as to determine an optimum gradient P for performing the calculation of the distance R. The filtering may for example consist in selecting the minimum gradient from among the N−1 gradients P_(p). As an alternative, the filtering may consist in selecting a plurality of gradients from among the N−1 gradients P_(p), then averaging the selected gradients. As an alternative, the filtering may consist in averaging the N−1 gradients P_(p). Other types of filtering may of course be contemplated.

Next, in a step Cal_R, the distance R separating the vehicle V and the identifier I is calculated by the computer Xv of the vehicle V, using the following formula: R=c/2π*P.

On the basis of the calculated distance R and depending on a specific requested function (opening of a door, closure of a door, starting of the vehicle, for example), the computer Xv of the vehicle V is able to determine whether or not the function should be performed.

Naturally, the steps of the method could, as an alternative, be performed in another technically feasible order than the one presented above. 

The invention claimed is:
 1. A method for measuring a distance R separating a vehicle (V) and an identifier (I) for accessing and starting the vehicle, the vehicle and the identifier being synchronized, the method comprising: N sequences Sq_(p), p∈[1;N], N being a natural number at least greater than 3, each sequence Sq_(p) comprising the following steps: transmission, from a transmitter of the vehicle to a receiver of the identifier, of a first signal of frequency f_(p); measurement, by a computer of the identifier, of a phase ϕ_(vip), modulo 2π, of the received first signal, in relation to a second signal of phase ϕ_(ip)(t)=ϕ_(0ip) +2πf_(p)t; transmission, from a transmitter of the identifier to a receiver of the vehicle, of the second signal; calculation, by a computer of the vehicle, of a phase ϕ_(ivp), modulo 2π, of the received second signal, in relation to the first signal; calculation of the average ϕ_(p) of the phase ϕ_(vip) and of the phase ϕ_(ivp); for each value of p between 1 and N-1, calculation of a gradient P_(p) using the formula P_(p)=(ϕ_(p+1)−ϕ_(p))/(f_(p+1)−f_(p)); filtering of the N-1 values of calculated gradients P_(p) so as to determine an optimum gradient P; and calculation of the distance R on the basis of the optimum gradient P.
 2. The method as claimed in claim 1, wherein the distance R is calculated, during the calculation step, using the following formula: R=−c/(2π)*P where c is the speed of propagation of the first signal and of the second signal.
 3. The method as claimed in claim 1, wherein the N sequences Sq_(p), p∈[1;N] are such that, for all values of p between 1 and N-1, f_(p+1)−f_(p) is constant.
 4. The method as claimed in claim 1, wherein the filtering comprises selecting a plurality of gradients from among the N gradients P_(p) calculated during the calculation steps.
 5. The method as claimed in claim 4, wherein the filtering comprises averaging the plurality of selected gradients. 